The present invention is directed to gas discharge displays, and more particularly, to the method and means for introducing mercury vapor into the gas envelope of the display. Mercury vapor is introduced to minimize cathode sputtering which is a typical phenomenon. Displays can also be operated at a higher current level and they have a longer life when mercury vapor is utilized. However, it is undesirable to have liquid mercury in the display panel, for this may cause shorting of adjacent electrodes, the formation of local hot spots and a discoloration of the transparent viewing surfaces. It is a goal to provide mercury vapor to the display without introducing liquid mercury into the display.
The prior art has numerous references which teach various methods and means for introduction of mercury vapor to the display. One method is to encapsulate liquid mercury in a small glass capsule which is situated within the actual display. At the time when mercury vapor is desired, energy of some form is supplied to the display causing the capsule to rupture and release mercury vapor and liquid mercury into the display. Although this method does provide the desired mercury vapor, it has the disadvantage of introducing liquid mercury directly into the display.
Attention is directed to U.S. Pat. Nos. 3,828,218, hereinafter referred to as Fehnel, and 3,872,339, hereinafter referred to as Maloney. Fehnel provides means for the introduction of mercury vapor to the display panel and is described in the abstract. "The panel includes a tubulation secured to the base plate through which gas and mercury vapor are introduced into the panel through a hole in the panel base plate. A fine mesh nickel screen is disposed in the tubulation adjacent to the hole in the base plate to prevent an excess of free mercury from entering the panel and, at the same time, to provide a source of mercury by way of the mercury which amalgamates with the screen." The use of the nickel screen to prevent liquid mercury from entering the display causes many problems. The metal mesh can be a source of contamination which leads to contamination of the display. At elevated temperatures drops of liquid mercury can pass through the mesh screen and enter the display. The mesh is always semi-open and cannot completely close off the display. It is difficult to seal the metal mesh to a glass surface such as the base plate, and it is difficult to evacuate the display when the metal mesh is secured to the base plate since the evacuation must occur through the actual mesh screen.
Maloney is very similar to Fehnel and is described in the abstract. "The panel includes a tubulation secured to the base plate through which gas and mercury vapor are introduced into the panel through a hole in the panel base plate. A mass of fibrous material is disposed in the tubulation adjacent to the hole in the base plate to prevent globules of free mercury from entering the panel and, at the same time, permitting mercury vapor to enter the panel." Maloney suffers from all of the disadvantages of Fehnel. The fibrous material can be a source of contamination, and at elevated temperatures droplets of liquid mercury could pass through the fibrous material into the display. The fibrous material is always semi-open and cannot completely close off the display. Means must be employed to hold the fibrous material in place, and it is difficult to evacuate the display when the fibrous material is in place since the evacuation must proceed through the fibrous material.
The preceding cited patents provide means which allow the introduction of mercury vapor while attempting to limit the entrance of liquid mercury into the display. However, these patents are subject to problems and difficulties which the present invention seeks to overcome. A significant problem with prior arrangements is to make a hole small enough leading into the sealed envelope to prevent the entrance of liquid mercury. Typically a hole in the range of 1-2 mils is needed to prevent the entrance of liquid mercury. Consequently, most arrangements utilize some blocking mechanism over the hole.